Though the work of the Radio-Intercept and Goniometric Direction-Finding stations lacked in some measure the danger connected with that of the ground listening-posts, it nevertheless provided many interesting incidents in the life of the Signal Corps man. The function of radio-intercept stations is, as their name implies, the interception of enemy radio messages. Goniometric stations are used, on the other hand, for locating enemy radio-stations, the work being carried on on much the same principles as flash-ranging, which I have described at some length in another chapter. By placing a goniometer—an instrument for measuring angles—at each end of a base line of known length, it is a comparatively simple matter to ascertain the angle of direction of an enemy radio-station, and, by prolonging the lines of these angles until they intersect, the location of the station can be approximately determined. That done, the information was sent to the artillery, which proceeded to sweep the vicinity in which the radio-station was known to be with a hurricane of shell. So highly was this system of radio detection developed that, after the salient at St. Mihiel had been cleared of Germans, every radio-station which our Goniometric Service had located previous to the attack was verified, the greatest error in location being approximately 500 yards. In many cases some of the German wireless equipment was still in the dugouts, and much interesting printed matter was picked up. This was the first corroboration of the effectiveness of our Radio Intelligence work.